Chronic endemic respiratory disease of dairy calves and an important component of bovine respiratory disease complex. It primarily affects calves up to six months of age and the etiology is multifactorial. Stress plus a primary viral infection is followed by a secondary bacterial infection. The latter is most commonly associated with pasteurella multocida producing a purulent bronchopneumonia. Sometimes present are mannheimia haemolytica; haemophilus somnus and mycoplasma species.